10 U.S.C. § 332 : US Code - Section 332: Use of militia and armed forces to enforce Federal authority

      Whenever the President considers that unlawful obstructions,
    combinations, or assemblages, or rebellion against the authority of
    the United States, make it impracticable to enforce the laws of the
    United States in any State by the ordinary course of judicial
    proceedings, he may call into Federal service such of the militia
    of any State, and use such of the armed forces, as he considers
    necessary to enforce those laws or to suppress the rebellion.